This invention relates to thermoplastic sacks formed of uni-axially oriented linear polyethylene films and, more particularly, to both tubular and heavy duty shipping sacks.
Thermoplastic sacks are used in the packaging, transportation or storage of a great variety of materials ranging from powders and granules, bulky and lightweight materials, and agricultural materials such as hay and silage. The thermoplastic sacks according to this invention have general applicability to such products.
Bulky but lightweight materials such as fiberglass insulation and peat moss are generally shipped in compressed form in thermoplastic sacks. These sacks are generally known as tubular insulation sacks or bags and take the form of an extended envelope or tube sealed at one end prior to its being filled with product. For the most part these sacks are produced by the commonly known in the art "blown film" process, which owes its popularity to the fact that it can be quickly and readily adapted to the production of different widths and thicknesses of continuous tubes which can then be easily cut to length and sealed at one end to produce an open top sack.
It will be readily appreciated that the thinner the film thickness (gauge), commensurate with acceptable film properties, the less the amount of thermoplastic material required. This downgauging of sack wall thicknesss is a most desirable industrial goal. Walls of sacks produced as tubes by the blown film process, typically, have a film thickness in the range of 3-6 mil (75-150.times.10.sup.-4 cm) which is generally determined by the machine direction (M.D.) tensile strength necessary to handle the package weight, the film stretch resistance required to prevent expansion of the compressed product and the puncture resistance of the bag for distribution handling. The tubes from which these sacks are commonly made are produced with a bubble diameter/die diameter generally of 3:1 in order to optimize film strength properties.
Although various attempts have been made to use high density polyethylene for the manufacture of downgauged bags because of its high stretch resistance and tensile strength these have largely been abandoned because of poor tear resistance and puncture properties. In view of this, polyethylene insulation sacks are most commonly made from resins which have superior tear resistance and puncture properties such as low density or linear low density polyethylene.
It is well known in the art to produce polyethylene films having enhanced puncture, tensile strength and stretch resistance by the process of uni-axially cold drawing the film below its melting point. However, because of the unbalanced physical properties e.g. poor M.D. tear strengths, of these oriented films, which causes "splittiness", they have been ignored for use in tubular shipping sacks.